1. Field of the Invention
The present invention relates to a method and apparatus for reviewing defects on a mask.
2. Background Art
In a process for manufacturing a semiconductor device, a reticle or photomask (hereinafter referred to as a mask) is used to form a circuit pattern on a wafer. When the mask has a defect, the defect is transferred to the circuit pattern. Thus, the mask needs to be defectless. In response to such needs, defects on a formed mask are inspected by comparing an optical image of the mask with a standard image.
A die-to-die inspection and a die-to-database inspection are known in the art as a method for inspecting defects on a mask. In the die-to-die inspection, an optical image of a pattern that is written on a region of a mask and has a certain shape is compared with an optical image of a pattern that is written on another region of the mask and has the same shape. In this case, one of the optical images is regarded as a standard image. That is, a difference between the standard image and the other optical image is compared and detected. Therefore, a defect detected by the die-to-die inspection also is included in defects detected by comparing the standard image with the other optical image. By contrast, in the die-to-database inspection, a reference image (standard image) created based on design data (CAD data) used to form a mask is compared with an optical image of a pattern written on the mask.
It is known that an operator performs mask inspection review (hereinafter abbreviated to review) to finally determine as to whether a defect detected by comparing an optical image with a standard image is an actual defect or a pseudo defect (for example, Japanese Patent Laid-open No. 2005-134347).
The term pseudo defect device those not determined to be actually defective among the results detected as a defect by a mask inspection apparatus. The pseudo defect may be generated due to malfunctioning of the mask inspection apparatus, distortion on the surface of the mask, or a variation in an environment (temperature, humidity, atmospheric pressure, etc.) during the inspection of the mask.
With higher integration of semiconductor devices and smaller sizes of the semiconductor devices in recent years, the amount of design data used to form a mask tends to be increased. To reduce the amount of the design data, there has been proposed a design data format using an iteration expression to write patterns that have the same shape with each other and are placed at respective different locations. According to the design data format, the shape of the patterns and information on the locations at which the patterns are to be written are defined.
In an optical image of a mask created based on the design data using the iteration expression to write the patterns, four patterns 10 having the same shape are arranged at a pitch P1 as shown in FIG. 4. It should be noted that rectangular patterns shown in FIG. 4 are not written using the iteration expression in the design data.
As a result of a comparison by a mask inspection apparatus that compares the optical image with a standard image, if defects Da, Db and Dd are each detected at or near a corresponding respective location in three of the four patterns 10, as shown in FIG. 4, there is high expectation that review results of the Da, Db and Dd are the same.
In review described in Japanese Patent Laid-open No. 2005-134347, detailed review is performed to determine whether each of all defects detected by the comparisons is an actual defect or a pseudo defect. Thus, it takes much time to complete the review.